[arunraj]

Dear all,

I am simulation 2-D thermosyphon simulation using ANSYS Fluent 'Evaporation-condensation model'. I am getting some reasonable result with the literature available. However, I would like to know the experts opinion on setting evaporation and condensation model. For example, I found a paper which suggest that ratio of evaporation and condensation frequency needs to be ratio of liquid density to the vapor density. But it didn't yield any good result in my case. Any such ideas would be highly appreciated. I believe there are some good ways to guess these two values before simulation.

'Effects of mass transfer time relaxation parameters on condensation in a thermosyphon' Journal of Mechanical Science and Technology 29 (12) (2015) 5497~5505

[vinerm]

Setting of evaporation and condensation frequencies on the basis of density ratio would be absurd. These frequencies are tuning parameters and need to be tuned such that the values match with the experiments. Default values are very small and you may require values as high as 1e5 to match with data.

[arunraj]

Dear vinerm,

Thank you for your kind advice.

I can will start with the condensation frequency of 1000 and keep evaporation in 1 and gradually increase. However, the real problem is the time step size. I am usually getting the error global courant number greater than 250. Could you please let us know

1. How to effectively set the time step for high condensation frequencies? I have tried to use the adaptive time step but the results do not make much sense. Velocities are going in order of 1000 m/s. Is reducing fixed time step size to 10^-5 or -6 or -7 the only way forward? Then it would take few weeks to get some good result.

2. Also, I want to know what is the basis to set, 'From' and 'to' phase in mass transfer mechanism window if water vapor is the primary phase (phase 1) and water liquid is the secondary phase (phase 2). (Or) it will not play any role in the final result.

Thank you in advance.

[vinerm]

You can use Implicit VOF instead of Explicit VOF and then you can use a higher time-step. Explicit VOF always requires smaller time-step. Other option is to coarsen the mesh so that you could use larger time-step with Explicit VOF.

For Evaporation-Condensation, liquid should always be selected as From and vapor as To.

[arunraj]

Dear vinerm,

It works with explicit too with adaptive time.

Thank you for your advice. Please be active on this forum. You are doing a great job helping many.

Thank you once again.

[Vishsel]

Hello everyone,

My task is to setting up a case for condensation process using Ansys fluent and i don't have prior experience with multiphase modeling.. So please help me out for initiating this task..

i have

Enabled Mixture model, Is this is the correct model for my application ? (Please see below shown pic)

Phases as vapor for primary and liquid for secondary..

Phase interaction --> mass --> liquid to vapor
Mechanism Eva-cond --> 0.1 for both frequencies

Material Properties water-liquid std.state.enthalpy 0, water-vapor std.state.enthalpy 2.992325e+07.. Is it correct for my case ?

Boundary condition - massflow-inlet --> liquid 0, vapor 0.5 kg/s because of vapor is inlet. And it should condensate and give outlet as water.

Below shown picture is the task.

Thank you in advance
Vishsel